In recent years, as a power generation system which is mild to the global environment and clean, fuel cells have drawn attention. Fuel cells are classified into a phosphoric acid type, a molten carbonate type, a solid oxide type, a polymer electrolyte type, etc. Among them, polymer electrolyte fuel cells are tried to be applied as power sources for electric automobiles, power sources for portable apparatuses, and, further, applied to domestic cogeneration systems utilizing electricity and heat at the same time, from the viewpoints of workability at low temperatures, miniaturization and lightening, etc.
A polymer electrolyte fuel cell is generally composed as follows. First, on both sides of an electrolyte membrane having proton conductivity, catalyst layers comprising a platinum group metal catalyst supported on carbon powder and an ion-conducting binder (the ions are usually protons) comprising a polymer electrolyte are formed, respectively. On the outsides of the catalyst layers, gas diffusion layers as porous materials through which fuel gas and oxidant gas can pass are formed, respectively. As the gas diffusion layers, carbon paper, carbon cloth, etc. are used. An integrated combination of the catalyst layer and the gas diffusion layer is called a gas diffusion electrode, and a structure wherein a pair of gas diffusion electrodes are stuck to the electrolyte membrane so that the catalyst layers can face to the electrolyte membrane, respectively is called a membrane electrode assembly (MEA). On both sides of the membrane electrode assembly, separators having electric conductivity and gastightness are placed. Gas passages supplying the fuel gas or oxidant gas (e.g., air) onto the electrode surfaces are formed, respectively, at the contact parts of the membrane electrode assembly and the separators or inside the separators. Power generation is started by supplying a fuel gas such as hydrogen gas or methanol to one electrode (fuel electrode) and an oxidant gas containing oxygen such as air to the other electrode (oxygen electrode). Namely, the fuel gas is ionized at the fuel electrode to form protons and electrons, the protons pass through the electrolyte membrane and transferred to the oxygen electrode, the electrons are transferred via an external circuit formed by connecting both electrodes into the oxygen electrode, and they react with the oxidant gas to form water. Thus, the chemical energy of the fuel gas is directly converted into electric energy which can be taken out.
Further, in addition to such proton exchange-type fuel cells, anion exchange-type fuel cells using an anion-conducting membrane and an anion-conducting binder (the anions are usually hydroxide ions) are also studied. It is known that in anion exchange-type fuel cells, overvoltage at the oxygen electrode is reduced, and the improvement of energy efficiency is expected. Further, it is said that, when methanol is used as fuel, methanol crossover wherein methanol passes through the electrolyte membrane between the electrodes is reduced. The constitution of a polymer electrolyte fuel cell in this case is basically the same as in the proton exchange-type fuel cell except that an anion-conducting membrane and an anion-conducting binder are used in place of the proton-conducting membrane and the proton-conducting binder, respectively. As to the mechanism of generation of electric energy, oxygen, water and electrons react at the oxygen electrode to form hydroxide ions, the hydroxide ions pass through the anion-conducting membrane and react with hydrogen at the fuel electrode to form water and electrons, and the electrons are transferred via an external circuit formed by connecting both electrodes into the oxygen electrode and react again with oxygen and water to form hydroxide ions. Thus, the chemical energy of the fuel gas is directly converted into electric energy which can be taken out.
In both proton exchange-type fuel cells and anion exchange-type fuel cells, the above electrode reactions take place at the boundary of the three phases of the catalyst surface, the fuel gas or oxidant gas and the ion-conducting binder as a polymer electrolyte, with the catalyst layer as a reaction site. The ion-conducting binder is used for the purpose of binding the catalyst and heightening the utilization efficiency of the catalyst by mediating the transfer of protons or hydroxide ions from the catalyst layer to the electrolyte membrane. Therefore, catalyst particles not covered with the ion-conducting binder cannot take part in the formation of the three-phase boundary, and it is hard for such particles to contribute to the reaction. Further, in order to obtain high efficiency, the minute structural design of the catalyst layer including pore structure for diffusing fuel gas or oxidant gas, the dispersion state of the catalyst, etc. becomes important. Further, at the gas diffusion electrode part, there arises a case where the catalyst surface is covered with water contained in the reaction gas or formed at the oxygen electrode and the fuel gas or oxidant gas cannot contact with the catalyst surface, and as a result, power generation is stopped, or a case where such water prevents the fuel gas or oxidant gas from being supplied or discharged to stop the electrode reaction. Therefore, the water repellency of the gas diffusion electrode part is required.
As to the joint of the gas diffusion electrode and the electrolyte membrane, a method wherein the joint is carried out by hot press is known. However, since it is hard to obtain good joint strength and good electric contact state by mere hot press, it is preferred to heighten the adhesion between the gas diffusion electrode and the electrolyte membrane by applying an ion-conducting binder as an adhesive resin on the catalyst layer surface of the gas diffusion electrode. In such application, the ion-conducting binder is used generally in a solution sate.
As ion conducting binders used in proton exchange-type fuel cells (cation-conducting binders), Nation (registered trade mark of Dupont Co., as is the same hereinafter) which is a perfluorosulfonic acid-type polymer as mentioned in JP-B-2-7398 and JP-A-3-208260 is generally used because it is chemically stable. However, Nation is a fluoropolymer and very expensive. Further, fluorine-containing polymers contain fluorine and consideration to the environment is necessary at the time of synthesis and disposal.
In view of these problems, cation-conducting binders in place of fluoropolymers have been developed. For example, aromatic engineering plastic type resins in which an ion-conducting group such as a sulfonic acid group is introduced is studied. Besides aromatic engineering plastic-type resins, cation-conducting binders comprising styrene type thermoplastic elastomers have been proposed (JP-A-2002-164055 and JP-Tokuhyo-2000-513484). For example, sulfonated SEES (SEBS is an abbreviation of polystyrene-poly(ethylene-butylene)-polystyrene tri-block copolymer) is proposed, and it is disclosed that it is sparingly soluble in water, and has good joint strength with an electrolyte membrane.
On the other hand, as anion-conducting binders used in anion exchange-type fuel cells, one wherein the sulfonic acid groups of perfluorosulfonic acid-type polymers are converted to anion exchange groups is known. For example, one obtained by copolymerizing tetrafiuoroethylene with CF2═CFOCF2CF(CF3)O(CF2)2SO3F and then making N,N,N′-trimethylethylenediamme act thereon is disclosed in JP-A-2000-331693 (paragraph 0026). However, this ion-conducting binder has the same drawback as Nafion. As another anion-conducting binder, an anion exchange resin obtained by chroromethylating a copolymer of an aromatic polyethersulfone with an aromatic polythioethersulfone and then aminating the resulting compound is disclosed in JP-A-11-273695 (paragraph 0046). As still another anion-conducting binder, an anion-conducting binder obtained by introducing a quaternary ammonium group into a polystyrene-poly (ethylene-butylene)-polystyrene tri-block copolymer or the like is disclosed in JP-A-2002-367626 (paragraphs 0041 and 0045).